Modeling the Evolution of Crocodyliform Cranial Shape Through Time

Abstract

Crocodyliforms have a rich fossil record, attained global distribution, and filled a wide variety of ecological niches throughout their history. The group has explored a broad range of cranial morphologies through time, particularly in relation to rostral and supratemporal fossa (STF) form. These character complexes are functionally related, with the snout acting as the prey capture apparatus and the STF as the origin for the pseudotemporalis and/or adductor mandibulae externus profundus musculature. A correlation between STF size and snout shape has been previously suggested (e.g. taxa with long slender snouts tend to have larger STF, possibly related to the importance of the aforementioned muscles in their prey capture strategy). To investigate evolutionary patterns of these cranial modules we quantified snout and supratemporal fossa shape using sliding semilandmark‐based geometric morphometric methods. We then modeled the evolution of these modules across a time‐calibrated phylogeny of Crocodyliformes, testing for differences in allometric patterning (using new functions in the R package bayou and phylogenetic ANCOVA as implemented in evomap). Our results indicate a significant shift in the covariation of STF relative to snout length at the origin of Eusuchia. In comparison with non‐eusuchian crocodyliforms, eusuchians exhibit less change in STF size relative to snout length. This shift in phenotypic covariation is potentially related to a change in pattern (but not strength) of developmental integration between these modules.As these modules are related to feeding, this shift in phenotypic covariation may indicate an important transition in the evolution of the crocodyliform feeding apparatus at the origin of Eusuchia. That non‐eusuchian crocodyliforms tend to have a larger STF suggests a reliance on muscles occupying the dorsal temporal fenestra, while the relatively smaller STF of eusuchians may mark a shift to an increased role of the pterydoideus musculature responsible for the extreme bite forces of extant crocodylians. If such a model holds true, it may serve to best explain the observed shift in covariation between these modules. The timing of this transition in our analysis matches closely with a previous study modeling the evolution of bite force in the group based on a different osteological proxy. Collectively this suggests that the modern crocodylian feeding apparatus developed around the origin of the crown group, coincident with the establishment of the eusuchian secondary palate.Support or Funding InformationNational Science Foundation grant NSF‐DEB 1754596This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.